Total guess, but I am going to say anywhere from 4-6. Open some up and try it out?
Edit: By the way, totally unrelated but I seen it earlier, so I thought I would warn you, you have been reported to be a scammer- http://sythe.org/showthread.php?t=1485500
So you might want to look into defending yourself if you are not, or just leaving if you are.
It depends on how you run the bots. What script you are using? Will it memory leak? You'd be very VERY lucky to run 6. You would even be pushing it with 4.